STAT3 is the acronym for signal transducer and activator of transcription 3. As the name implies, STAT3 is a transcription factor and, in humans, it is encoded by the gene appropriately known as the STAT3 gene.
STAT3 is a protein which belongs in a family of related STAT proteins. STAT proteins are phosphorylated in response to various growth factors and cytokines which are associated with cell receptors. The phoshorylated STAT proteins then form either homo or heterodimers which move into the cell nucleus to function as transcription activators. Growth factors and cytokines which are known to activate STAT proteins include IL5, IL6, LIF, IFNs, EGF and BMP2.
STAT3 is known to be responsible for mediating expression of many genes when the cell is exposed to stimuli. Accordingly, STAT3 has a central role in important cellular processes involving cell growth and death, apoptosis.
Presence of constitutive STAT3 is found in a number of human cancers and has also been found to be an indicator of poor outcome. Constitutive STAT3 promotes cancerous cell growth by being anti-apoptotic and also stimulating uncontrolled cell proliferation.
Suppression of constitutive STAT3 protein activation in human malignancies represents an important target for molecular therapeutic intervention. STAT proteins mediate the relay of extracellular signals from various cell surface protein receptors to the nucleus, where they help to initiate and regulate specific anti-apoptotic and cell survival gene expression. In part cular, the STAT3 protein isoform is known to directly up-regulate Bel-XL, c-Myc, Mel-I, VEGF and cyclin DI/D2, contributing directly to compromised cellular regulation by stimulating cell proliferation and preventing apoptosis in numerous human cancers.
STAT3 activation occurs via phosphorylation of tyrosine 705, which promotes STAT dimerformation through reciprocal STAT phosphotyrosine-SH2 domain interactions.” STAT dimers then translocate to the nucleus, where they regulate unique gene expression programs through interaction with specific DNA response elements. STAT3 targeted gene expression confers resistance to apoptosis in many tumor cells. and promotes cell survival, contributing to the resistance of these cancers to currently available chemotherapeutics. Successful STAT3 inhibitors may thus be used to sensitize human cancers with constitutively active STAT3 to existing chemotherapeutic agents, potentially reducing the side effects associated with conventional, aggressive chemotherapy.
Despite the difficulties in identifying protein surface-recognition agents, the promise of STAT3 modulators warrants investigation. Successful peptidic and nonpeptidomimetic small molecules that are capable of targeting malignant cell lines with constitutively activated Stat3 protein are limited to a few examples that include Stattic, STA-21, and S3I-201, which were all identified through high-throughput virtual or biochemical screening approaches. Our first-generation designs were simple peptidomimetics derived from the natural sequence, of which ISS610 was the most potent (see FIG. 1). More recently we have discovered S3I-M2001 (compound 15, Table 1) an oxazole-based small-molecule inhibitor that shows promising inhibition of Stat3 function, and we herein report a family of rationally designed small-molecule, nonpeptidic Stat3 inhibitors.
These agents inhibit Stat3 protein dimerization and induce apoptosis in Stat3-transformed cells and Stat3-dependent breast oncogenic cell lines. The crystal structure of the Stat3-SH2 domain reveals a shallow triangular pocket that is composed of two hydrophobic sites and a hydrophilic phosphate-recognition pocket. Docking studies on our initial lead, peptidomimetic, ISS610, showed that only the hydrophobic pockets in the binding domain were effectively occupied (See FIG. 1 A). To address this problem we theorized that trisubstituted heterocyclic scaffolds, such as oxazoles and thiazoles, might effectively access all three sites (FIG. 1B).